Search Results (152 total)

We have obtained mid-infrared optical absorption spectra of the S=1/2 quasi-one-dimensional CuO using polarized transmission measurement and interpreted the spectra in terms of phonon-assisted magnetic excitations. When the electric field is parallel to the main antiferromagnetic direction a Δ shaped peak is observed with the maximum at ω=0.23 eV which is attributed to spinons along Cu-O chains. At low temperatures in the antiferromagnetic phase another peak appears at ω=0.16 eV which is attributed to two-magnon absorption but the spinon peak remains. This behavior is interpreted as due to a dimensional crossover where the low-temperature three-dimensional magnetic phase keeps short-range characteristics of a one-dimensional magnet.

We report on the observation of weak antilocalization associated with large spin-orbit coupling in a tunable d electron system: a quasi-two-dimensional electron gas formed in a KTaO₃ field-effect transistor. We find that spin-precession length of electrons can be tuned by gate voltage V_G and is as short as tens of nanometers at large V_G. Our results show that 5d transition-metal compounds having strong atomic spin-orbit couplings induced by heavy 5d elements could be utilized to electrically manipulate spins in nanoscale spintronic devices.

The effects of electric and magnetic fields on magnetic and electric properties have been investigated for a triangular lattice antiferromagnet CuCrO₂ showing magnetically induced ferroelectric order. We demonstrate that ferroelectric polarization reversal can be finely tuned by using both magnetic and electric fields in the triangular lattice antiferromagnet. The observed magnetoelectric tunability can be attributed to small in-plane spin anisotropy and a resultant high degree of freedom for the direction of ferroelectric polarization, which is characteristic of a multiferroic triangular lattice antiferromagnet with out-of-plane 120° spin structure.

The dynamics of a MeV laser-produced proton beam affected by a radio frequency (rf) electric field has been studied. The proton beam was emitted normal to the rear surface of a thin polyimide target irradiated with an ultrashort pulsed laser with a power density of 4×10¹⁸  W/cm². The energy spread was compressed to less than 11% at the full width at half maximum (FWHM) by an rf field. Focusing and defocusing effects of the transverse direction were also observed. These effects were analyzed and reproduced by Monte Carlo simulations. The simulation results show that the transversely focused protons had a broad continuous spectrum, while the peaks in the proton spectrum were defocused. Based on this new information, we propose that elimination of the continuous energy component of laser-produced protons is possible by utilizing a focal length difference between the continuous spectral protons and the protons included in the spectral peak.

We investigated the ultrafast dynamics of photoinduced melting of charge and orbital order in a perovskite-type manganite, Nd_0.5Ca_0.5MnO₃, by means of pump-probe reflection spectroscopy. By irradiation with a 20-fs laser pulse, only the charge sectors become melted within ∼30 fs through strong electron correlations. During this process the oxygen displacements associated with the ordered phase persist. These displacements are subsequently released accompanied by several coherent oscillations. The coherent oscillation corresponding to the Jahn-Teller mode has relatively large amplitude demonstrating that the orbital order plays a significant role on the stabilization of the charge order.

There exist new polarized structure functions in a spin-one hadron. In deep inelastic electron scattering from a spin-one hadron, there are eight structure functions F₁, F₂, g₁, g₂, b₁, b₂, b₃, and b₄. We derive projections to these eight functions from the hadron tensor W^μν by combinations of the hadron momentum and its polarization vectors.

The correct stacking of hexagonal layers is used to obtain accurate estimates for the exchange and anisotropy parameters of the geometrically frustrated antiferromagnet CuFeO₂. Those parameters are highly constrained by the stability of a collinear metamagnetic phase between fields of 13.5 and 20 T. Constrained fits of the spin-wave frequencies of the collinear ↑↑↓↓ phase below 7 T are used to identify the magnetic unit cell of the metamagnetic ↑↑↑↓↓ phase, which contains two hexagonal layers and 10 Fe³⁺ spins.

We have grown single crystals of a triangular lattice antiferromagnet (TLA), CuCrO₂, and investigated the correlation between magnetic and dielectric properties. Two magnetic phase transitions are observed at T_N2≈24.2 K and T_N1≈23.6 K. It was found that ferroelectric polarization along the triangular lattice plane develops at T_N1, suggesting that the system undergoes a transition into an out-of-plane 120° spin-chiral phase at T_N1. The TLA provides an opportunity for unique magnetoelectric control of spin-chiral ferroelectric domain structures by means of electric and/or magnetic fields.

We report the detection of a magnetic resonance mode in multiferroic Ba_0.6Sr_1.4Zn₂Fe₁₂O₂₂ using time-domain pump-probe reflectance spectroscopy. Magnetic sublattice precession is coherently excited via picosecond thermal modification of the exchange energy. Importantly, this precession is recorded as a change in reflectance caused by the dynamic magnetoelectric effect. Thus, transient reflectance provides a sensitive probe of magnetization dynamics in materials with strong magnetoelectric coupling, such as multiferroics, revealing new possibilities for application in spintronics and ultrafast manipulation of magnetic moments.

Temperature dependence of spin relaxation process in a Cu wire has been studied by means of nonlocal spin-valve measurements. The spin-diffusion length of the Cu wire is found to take maximum at the characteristic temperature, below which the spin-diffusion length is reduced. The mechanism of the reduction can be explained by considering the spin-flip scattering due to the oxidized surface of the Cu wire. The thickness dependence of the characteristic temperature supports the interpretation with the surface oxidation.

We present a remarkably accurate method for determining the wave field of an x-ray nanobeam. The intensity profile of a beam was directly measured over a range of three orders of magnitude while its phase distribution was successfully recovered using an iterative algorithm. The evolution of the wave field along the beam propagation direction was precisely simulated, and there was good agreement with the experimental results.

Duration-controlled amplified spontaneous emission with an intensity of 10¹³  W∕cm² is used to convert a 7.5-μm-thick polyimide foil into a near-critical plasma, in which the p-polarized, 45-fs, 10¹⁹-W∕cm² laser pulse generates 3.8-MeV protons, emitted at some angle between the target normal and the laser propagation direction of 45°. Particle-in-cell simulations reveal that the efficient proton acceleration is due to the generation of a quasistatic magnetic field on the target rear side with magnetic pressure inducing and sustaining a charge separation electrostatic field.

We have studied the evolution of the spin Hall effect (SHE) in the regime where the material size responsible for the spin accumulation is either smaller or larger than the spin diffusion length. Lateral spin valve structures with Pt insertions were successfully used to measure the spin absorption efficiency as well as the spin accumulation in Pt induced through the spin Hall effect. Under a constant applied current the results show a decrease of the spin accumulation signal is more pronounced as the Pt thickness exceeds the spin diffusion length. This implies that the spin accumulation originates from bulk scattering inside the Pt wire and the spin diffusion length limits the SHE. We have also analyzed the temperature variation of the spin Hall conductivity to identify the dominant scattering mechanism.

A large spin accumulation due to the electrical spin injection has been observed in Permalloy-silver lateral spin-valve structures. The observed resistance change is the largest among the reported metallic lateral spin valves with Ohmic junctions. The spin-diffusion length deduced from the experimental results is also found to be the longest among the normal metals reported so far. All the results can be quantitatively explained by the common spin-diffusion model without any discrepancies unlike the results of Godfrey and Johnson.

Vertical stacks of two Co rings with deep submicron lateral sizes and different thicknesses are fabricated and studied. The experimental results suggest the existence of a metastable domain wall structure revealed by the micromagnetics simulation. The technologically important vortex-vortex switching is found dominated by the annular Oersted field of perpendicularly injected dc, but also significantly affected by the spin-transfer torque. The efficiency of the spin-transfer torque and the switching process are discussed.

We are able to continuously change the direction of polarization of spin accumulation in a nonmagnetic metal by varying the currents injected by two ferromagnetic spin injectors. From measurements made at a distance from the injection area, we find a cos⁡ϕ variation of the spin signal. This confirms that the angle of polarization of the nonlocal spin polarization with respect to the magnetization of the fixed spin detector is continuously varied as we change the injection currents. We give an explanation for the origin of this simple cos⁡ϕ variation of the spin signal.

The spin-wave excitations of the geometrically frustrated triangular lattice antiferromagnet CuFeO₂ have been measured using high resolution inelastic neutron scattering. Antiferromagnetic interactions up to third nearest neighbors in the ab plane (J₁, J₂, J₃, with J₂/J₁≈0.44 and J₃/J₁≈0.57), as well as out-of-plane coupling (J_z, with J_z/J₁≈0.29) are required to describe the spin-wave dispersion relations, indicating a three-dimensional character of the magnetic interactions. Two energy dips in the spin-wave dispersion occur at the incommensurate wave vectors associated with multiferroic phase and can be interpreted as dynamic precursors to the magnetoelectric behavior in this system.

We present extended x-ray-absorption fine structure (EXAFS) data at the Mn  K and Tb  L₃ edges that provide upper limits on the possible displacements of any atoms in TbMnO₃. The displacements must be less than 0.005–0.01  Å for all atoms, which eliminates the possibility of moderate distortions (0.02  Å) with a small c-axis component, but for which the displacements in the ab plane average to zero. Assuming the polarization arises from a displacement of the O2 atoms along the c axis, the measured polarization then leads to an O2 displacement that is at least 6×10⁻⁴  Å, well below our experimental limit. Thus, a combination of the EXAFS and the measured electrical polarization indicate that the atomic displacements likely lie in the range 6×10⁻⁴–5×10⁻³  Å.

In a plasma wake wave generated by a high power laser, modulations of the electron density take the shape of paraboloidal dense shells, moving almost at the speed of light. A counterpropagating laser pulse is partially reflected from the shells, acting as relativistic flying mirrors, producing a time-compressed frequency-multiplied pulse due to the double Doppler effect. The counterpropagating laser pulse reflection from the plasma wake wave accompanied by its frequency multiplication (with a factor from 50 to 114) was detected in our experiment.

We present a ^63,65Cu and ²⁹Si NMR study of the quasi-2D coupled spin 1/2 dimer compound BaCuSi₂O₆ in the magnetic field range 13–26  T and at temperatures as low as 50  mK. NMR data in the gapped phase reveal that below 90  K different intradimer exchange couplings and different gaps (Δ_B∕Δ_A=1.16) exist in every second plane along the c axis, in addition to a planar incommensurate (IC) modulation. ²⁹Si spectra in the field induced magnetic ordered phase reveal that close to the quantum critical point at H_c1=23.35  T the average boson density n̅ of the Bose-Einstein condensate is strongly modulated along the c axis with a density ratio for every second plane n̅ _A∕n̅ _B≃5. An IC modulation of the local density is also present in each plane.

The superconducting properties of filled skutterudite La_0.8Rh₄P₁₂ synthesized at 9.4 GPa and 1473 K using a belt-type apparatus were investigated by measuring the magnetization, electrical resistivity, and specific heat. Filled skutterudite La_0.8Rh₄P₁₂ is a type-II superconducting material with a critical temperature for the superconducting transition, T_C, of 14.9 K. The upper critical field H_C2(0) and Ginzburg-Landau coherent length ξ(0) were determined to be 167(1) kOe and 4.41(1) nm, respectively. The electronic heat-capacity coefficient γ_n and the Debye temperature Θ_D were 25.1(8) mJ∕mol K² and 459(12) K, respectively. Using T_C and Θ_D, the electron-phonon coupling constant λ was estimated to be approximately 0.7, suggesting that La_0.8Rh₄P₁₂ is an intermediately coupled superconducting material.

Reversible spin Hall effect comprising the direct and inverse spin Hall effects was electrically detected at room temperature. A platinum wire with a strong spin-orbit interaction is used not only as a spin current absorber but also as a spin-current source in the specially designed lateral structure. The obtained spin Hall conductivities are 2.4×10⁴  (Ωm)^-1 at room temperature, 10⁴ times larger than the previously reported values of semiconductor systems. Spin Hall conductivities obtained from both the direct and inverse spin Hall effects are experimentally confirmed to be the same, demonstrating the Onsager reciprocal relations between spin and charge currents.

Optical manipulation of the magnetic anisotropy is demonstrated for bilayered manganites, La_2-2xSr_1+2xMn₂O₇, by means of femtosecond Kerr-rotation measurements. Upon the photoexcitation on the x=0.32 crystal, the magnetization exhibits the precessional motion for about 1 ns, revealing the directional change of the magnetocrystalline anisotropy from the c axis to the ab plane. This change of the anisotropy induces the nonthermal decrease of the c-axis magnetization component for about 1 ns.

Photocapacitance and excitation photocapacitance methods were applied to reveal the dislocation-induced deep levels in coalescent epitaxial lateral overgrowth layers of InP. Point-contact Schottky barrier junctions with small junction areas were formed on dislocated and dislocation-free regions by using wedge wire-bonding of Au, and photocapacitance measurements were then carried out at 30 K. In the dislocation-free layers, the dominant deep level was located at 1.30 eV below the conduction band, whereas in the dislocated area, dominant deep levels were detected at 0.86 eV (λ=1.44 μm) and 1.05 eV (λ=1.18 μm) below the conduction band. A neutralized state was also detected at 0.66 eV above the valence band. From the detailed excitation photocapacitance results, it is shown that the defect configuration coordinate diagram of the dislocation-induced deep levels was considered with large Frank-Condon shifts (d_FC) of 0.28 eV. This means that the atomic configurations around the deep levels are highly relaxed, as expected from the structures of the dislocation cores.